Internal combustion engine systems including at least one turbocharger have long been used to increase the power and efficiency of the engine. The turbocharger includes a turbine operated by exhaust gas and a compressor operatively connected to the turbine for compressing intake air. Turbocharged engines rely upon the power from the turbocharger turbine to supply energy to the compressor. The compressor is typically connected to the turbine by a shaft and supplies energy to the engine charge air. During operation, exhaust gas flows from engine cylinders through exhaust valves to the turbine. The exhaust gas drives the turbine thereby causing the compressor to increase the flow of charge air into an engine cylinder thus permitting an increase in engine power output. An aftercooler may be provided between the compressor output and engine to further increase the quantity of charge air delivered to the engine cylinder.
If the engine is running at or near full throttle conditions, the turbine supplies considerable energy to the compressor. If, under these conditions, the fueling is suddenly reduced, the turbine inlet temperature decreases resulting in a decrease in power from the turbine. Consequently, the compressor receives less power from the turbine and begins to reduce speed. The pressure increase that the compressor is capable of delivering to the charge air depends substantially on the compressor speed and slightly on the flow rate of charge air through the compressor. As a practical matter during operation, the turbocharger speed decreases when the fuel is shut off. If the turbocharger speed decreases too rapidly, or the intake manifold pressure cannot decrease fast enough, the compressor may begin to undesirably experience "surge". If an aftercooler is provided, the aftercooler and associated piping volume contains a considerable amount of air which must be ingested by the engine before the pressure in the intake system can decrease. Therefore, the intake manifold pressure may not decrease at a sufficient rate due to the inability of the aftercooler volume to empty through the engine fast enough.
When the compressor begins to experience surge, the flow through the compressor alternates direction and magnitude rapidly, disadvantageously resulting in the generation of objectionable noise. The alternating flows also cause alternating axial forces on the turbocharger bearings possibly resulting in excessive bearing wear and damage.
Japanese patent application 5263671 discloses a valve timing control device for an internal combustion engine equipped with a turbocharger. The exhaust valves are controlled so that one valve opens during the exhaust stroke in advance of the opening of the remaining exhaust valve associated with the same cylinder. However, this reference does not appear to suggest opening an exhaust valve early so as to address the problem of surge.
International publication number WO 98/07973 discloses a turbocharged premixed charge compression ignition engine including exhaust valves. However, this reference fails to suggest any advantages of varying exhaust valve opening.
Consequently, there is a need for system and method for effectively enhancing turbocharger power and preventing compressor system surge during a decrease in fuel rate delivery to the engine.